We developed a mouse model of ileo-cecal resection (ICR) and used new and traditional approaches to define cellular mechanisms mediating adaptive intestinal growth to compensate for the intestinal loss. These mechanisms include a brief period of expansion of intestinal stem cells (ISC) immediately after ICR, followed by marked increases in crypt fission, and crypt number which mediate increases in mucosal surface area. Recent reports suggest that there may be two populations of ISC: a slowly cycling quiescent-ISC pool (q-ISC) located above Paneth cells (Upper Stem cell Zone, USZ) and a more rapidly cycling active-ISC pool (a-ISC) of crypt base columnar cells (CBC) marked by the expression of Lgr5. A complete understanding of the adaptive response will require analysis of these putative ISC sub-populations. Our preliminary studies demonstrate increased intestinal IGF-I expression during the period of ISC expansion after ICR. Glucagon-like peptide-2 (GLP-2) augmented the observed ISC expansion and local IGF-I expression, but only if given immediately following resection. In the clinical setting many patients are unable to tolerate enteral nutrition (EN) after massive intestinal loss, and require total parenteral nutrition (TPN). The increased incidence of chronic short bowel syndrome (SBS) in these patients suggests that TPN-feeding may attenuate the normal adaptive ISC expansion. The current proposal examines 3 hypotheses; 1. Following ICR, q- ISC and a-ISC show distinct kinetics, as recruitment of q-ISC will precede expansion of a-ISC. 2. IGF-I expression and signaling is required for normal ICR-induced expansion and mediates GLP-2 effects after ICR. Constitutive IGF-I expression will enhance and prolong ISC expansion following ICR. 3. EN drives IGF-I induced ISC expansion following ICR, but in TPN-fed mice GLP-2 will induce local IGF-I and restore ISC expansion following ICR. The following SA will test these hypotheses; SA1. Use the Lgr5-LacZ mouse to define whether ICR induces expansion of Lgr5/LacZ-positive a-ISC and how this relates to the process of crypt fission. Additional studies will validate the temporal expansion of a-ISC and q-ISC following ICR by defining the time course of label retention in the USZ and CBC regions after continuous thymidine analogue labeling. SA2. Study ISC expansion and crypt fission following ICR in transgenic mice with constitutive over-expression of IGF-I or impaired IGF-I signaling. Characterize expansion of ISC in mice with attenuated IGF-IR signaling following ICR + GLP-2. SA3. Randomize the route of providing nutrition to mice following ICR to EN 1 GLP-2 vs. TPN 1 GLP-2. Assess the influence of the route of nutrition and administration of GLP-2 on both intestinal expression of IGF-I and normal ISC expansion following resection.